Maximising power in optical communication networks

ABSTRACT

The relative launch power of signals at an add/drop node transponder is controlled to maximize available power. Known values of signal bandwidth and likely noise in the signal path, for example, derived from the system manager, are used to control the launch power into the optical amplifier, in order to optimize the launch power accordingly.

This invention relates to optical communications networks, and inparticular to optimising power available to launch signals onto opticalcommunications networks.

The design of photonics systems requires that the OSNR (optical signalto noise ratio) is above a given minimum value over the longest pathunder the worst case conditions.

Optical communication systems, such as wavelength division multiplex(WDM) systems typically use optical amplifiers in the signal path. Alimiting feature of optical amplifiers is the power available. This maybe due to safety constraints or cost. The maximum output power of anoptical amplifier is conventionally divided equally amongst all theoptical channels being transmitted.

However, higher bit rate channels such as 10 Gbit/s require a betterOSNR than lower bit rate channels such as 2.5 Gbit/s channels.

It has been proposed (EP-A-0 924 888) to adjust the optical power inindividual channels by measuring optical power along the transmissionpath in use. However, the processing overload for this is relativelyhigh.

Also, it has been proposed (WO 02/09299) to compensate for wavelengthdependent gain and noise profiles by pre-emphasising individual channelsof a WDM by values obtained by measuring the OSNR of the undistortedchannels.

The invention aims to maximise the power available to optical signalsrequiring a higher bandwidth and/or greater path length. Broadly, thisis achieved by dividing available output power amongst the opticalchannels according to their individual bandwidth/distance requirements.

More specifically, there is provided a method of controlling signallaunch power of at least one optical signal in an optical communicationsnetwork, comprising pre-distorting the launch power of the opticalsignal in accordance with a known value of the bandwidth of a modulationsignal used to modulate the optical signal.

The invention also provides apparatus for controlling signal launchpower of at least one optical signal in an optical communicationsnetwork, comprising a launcher for launching the optical signal onto thenetwork, and means for pre-distorting the launch power of the opticalsignal in accordance with a known value of the bandwidth of a modulationsignal used to modulate the optical signal.

This has the advantage that launch power of a first optical signal canbe conserved and redirected to a second optical signal whose associatedmodulation signal has a greater bandwidth than the modulation signalassociated with the first optical signal, without incurring a processingoverload due to making measurements.

Preferably, the optical communications network carries an n channelsignal multiplex, and a plurality of optical signals are launched from anetwork node.

Preferred embodiments have the advantage that for a given launch poweravailable at an add/drop node, the power can be distributed amongst thechannels in accordance with the requirements of each channel. This againcan increase the bandwidth that can be sent and increase thetransmission distance that can be achieved.

The noise is generated at the optical amplifiers, and the expected noisecan be determined knowing the route of the optical signal, that is, thenumber and type of optical amplifiers—the optical signal will passthrough in the network. This will be indicative of the OSNR. The knownvalues may be provided by management systems of the opticalcommunication systems, for example, by the network manager, or by ashelf manager. Equally, the known values may be provided by data passedalong a supervisory channel.

In a preferred embodiment, the pre-distorted optical signals are passedthough an optical amplifier, and the launch power is pre-distorted usinga comparator. A separate comparator is provided for each channel of theoptical multiplex, a suitable de-multiplexer being provided at theoutput of the optical amplifier. One input to the comparator is asignal, preferably electrical, derived from the output of the opticalamplifier for any particular optical channel, while the other isrepresentative of the known value of the bandwidth of the modulationsignal associated with the optical signal. The output of the comparatorcontrols the launch power of the optical signal for that channel intothe optical amplifier, for example, by means of a variable opticalattenuator for a through channel, or a transponder for an added channel.

An embodiment of the invention will now be described, by way of exampleonly, and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating the principle of theinvention;

FIG. 2 is a schematic diagram showing how launch power can be adjustedaccording to the connection path; and

FIG. 3 shows the invention applied to an optical ring network

The embodiments described divide the available output power amongst thechannels according to individual bandwidth requirements. The higher thebit rate, the higher the power level allocated to that channel ensuringthat all channels are launched such that they are received with adequateOSNR.

The power output for a given channel can also be controlled according tothe number and type of network elements the signal is to pass through.

Referring to FIG. 1, an optical amplifier 10 amplifies a WDM opticalsignal on a fibre 11. The output is split at a coupler 12 providing anoutput signal path 13 and a feedback control path 15. The feedbackcontrol path is demultiplexed by a demultiplexer 14 the outputs of whicheach have a pin diode 16 attached to convert the individual channelssignals into electrical signals. Only one is shown in FIG. 1 forsimplicity. The electrical signals are amplified by amplifier 18 andthen passed to a controller 22 the outputs of which are connected totransponder 20 or CCU 23 (a variable optical attenuator array) dependingon whether a channel is being added or passed through. In practice, theamplifier will amplify a number of channels, for example up to 32, butthere is a separate controller 22 for each channel. Each controller actsas a comparator, which compares the output of the amplifier 18,indicative of the power amplitude of the channel at the output of theoptical amplifier, with a reference level, chosen so that the output ofthe comparator, which is fed to the CCU or the transponder, produces alaunch signal of the desired power for that channel as input to theoptical amplifier.

In accordance with the invention, the launch power of each channel ispre-distorted in accordance with the known bandwidth of each channel(its bit rate) and in accordance with the noise expected to be added toeach signal in consequence of its routing.

The controller 22 receives the network and connectivity information 8required to assess the bandwidth and the routing from a shelf managementunit 24 under the overall control of network manager 9 and, knowing thepower of the channel, its bandwidth and the OSNR of the path, alters thereference at the comparator input to control the launched power of thechannel to optimise it. Thus, for example, it can be ensured that thesignal is not launched with too much power as this would waste a powerresource that could be directed to a higher bit rate channel.

Each channel may carry a different signal having a different bandwidthand therefore have a different launch power requirement. The systemmanager can evaluate each signal's power requirement for its knownbandwidth and routing. The system manager can then formulate a targetpower level for the controller to set the launched power level. Thus,the available launch power is not simply divided equally across thechannels but distributed according to a measure of channel powerrequirement, such as signal bandwidth and noise expected to be added,both as regards the number of optical amplification stages passedthrough and as regards the type of optical amplifier passed through.

Although the launch power is adjusted to take account of bit rate andsignal routing, it would be possible for the launch power to takeaccount of one of these only.

Turning now to FIG. 2, the circuit illustrated schematically shows howthe connection path can be determined. The figure illustrates a portionof an add/drop node which adds signals on to the network.

Thus, the add side of the add/drop node comprises a channel control unit30 to control through traffic and an add coupler 32 for addition ofsignals to the network. The signals to be added are provided from thetransmitters of node transponders 34 which output a number of signals onseparate channels, for example up to 32. These signals are multiplexedtogether by an optical multiplexer 36 and the signal multiplex addedonto the network by the coupler 32.

The output of the coupler is amplified by an optical amplifier 38 and asignal split from the main network path to derive a feedback signal by asplitter coupler 40. The coupler 40 has two outputs: a through output 42which is the main network path, and a drop path 44 from which thefeedback signal is derived. The drop path 44 carries the dropped signalmultiplex to a demultiplexer 46 where it is split into its componentchannels. A feedback signal is derived for each of the these channels,one only of which is shown in FIG. 2. The optical output of the channelfrom demultiplexer 46 is converted to an electrical signal by photodiode48, amplified by amplifier 50 and fed back to the controller block 22where the feedback signal is compared with a reference signal and acontrol output generated to adjust the transponder. The reference signalis derived from the signal bandwidth and routing information stored inblocks similar to blocks 8, 24 and 9 of FIG. 1, but not shown in FIG. 2,and used in the controller 22 as before.

The signal on the through path continues around the network until itreaches its destination, whereupon the signal is dropped to a receiver.Where a channel passes through a node the levelling controller blockwill control the amplitude of the channel for the current receivedrelative amplitude. The signal will pass through various stages ofprocessing depending on how long it stays on the network. Each of thesewill introduce an amount of noise onto the signal. In the example ofFIG. 2, the signal is shown as passing through three amplificationstages 52, 54, 56 by way of example. Each of these adds an element ofnoise N to the signal. Thus, the total signal noise will depend on theroute taken by the signal to its destination.

This signal is eventually dropped to the receiving node by a splitcoupler 58, filtered by a band pass filter 60 to isolate the signalchannel and passed to a receiver Rx 62 where it is converted to anelectrical signal for the user. The controller block 22 knows the noiseon the signal at the receiver, and the bandwidth of the signal, from theblocks similar to blocks 8,24 and 9 in FIG. 1, and can adjust the launchpower of the signal accordingly.

The transponder seeks to send the signal with just sufficient power toexceed the minimum OSNR thereby optimising power utilisation. As withthe previous example of FIG. 1, the controller 22 seeks to distributethe launch power to optimise the power for the bandwidth and paths ofthe signals.

FIG. 3 shows schematically how the network manager is attached to a ringnetwork 60, via gateway nodes 70, to manage the flow of traffic betweenthese nodes and other nodes 71. The various amplifiers between thenodes, such as amplifiers 52, 54 and 56 operate at a constant gain. Itis therefore possible for the network manager to determine the noisecontribution of each amplifier and to determined the signal path. Thus,the network manager can provide the add/drop node transponder launchingthe signal with an indication of the likely noise on the signal. Thenetwork manager will always be aware of the signal path, including, forexample, cases where a signal is sent round the longer of the two pathson a two fibre ring network, due to a component failure or the like onthe shorter path.

It will be appreciated from the above description that embodiments ofthe invention have the advantage of allowing the launch power for asignal to be chosen according to parameters of the signal such asbandwidth and signal path. This enables available launch power to bedistributed intelligently according to channel requirements. Moreover,it can enable other channels to be used at higher bandwidths or greaterdistances than would otherwise be possible.

Various modifications to the embodiments described are possible and willoccur to those skilled in the art without departing from the scope ofthe invention. For example, instead of using the network and shelfmanager to provide the expected routing to the block 8, this informationcould be provided by the supervisory channel, which could record thenumber and types of optical amplifiers which channels have passedthrough. In the case of FIG. 3, a knowledge of the type of amplifier tobe passed through by a signal propagating from gateway node 70 to node71, could be extracted from a signal which has passed through the node71 in the reverse direction on its way to the node 70, since the opticalamplifiers for each direction in a node will often be identical.

1-23. (canceled)
 24. A method of controlling signal launch power of at least one optical signal in an optical communication network, comprising the step of: pre-distorting the launch power of the at least one optical signal in accordance with a known value of a bandwidth of a modulation signal used to modulate the at least one optical signal.
 25. The method as claimed in claim 24, wherein the pre-distorting step is performed by pre-distorting the launch power of the at least one optical signal in accordance with a known value of expected noise on a signal path of the at least one optical signal.
 26. The method as claimed in claim 25, wherein the known values are provided by management systems of the optical communication network.
 27. The method as claimed in claim 25, wherein the known values are provided by a network and connectivity information unit.
 28. The method as claimed in claim 25, wherein the known values are supplied by a supervisory channel.
 29. The method as claimed in claim 24, wherein the at least one pre-distorted optical signal is passed through an optical amplifier.
 30. The method as claimed in claim 29, wherein the pre-distorting step is performed by using a comparator, which compares a signal derived from an output of the optical amplifier with a reference signal dependent on the known value of the bandwidth of the modulation signal used to modulate the at least one optical signal.
 31. The method as claimed in claim 25, wherein the known value for expected noise on the signal path of the at least one optical signal is derived from a knowledge of a number and a type of an optical amplifier through which the at least one optical signal will pass.
 32. The method as claimed in claim 24, wherein the optical communication network carries an n channel multiplex, and wherein the pre-distorting step is performed by an optical amplifier.
 33. The method as claimed in claim 24, wherein the launch power of the at least one optical signal with an associated modulation signal of a higher bandwidth is pre-distorted to increase a signal level of the at least one optical signal compared to an optical signal with an associated modulation signal of a lower bandwidth.
 34. The method as claimed in claim 25, wherein the launch power of the at least one optical signal is pre-distorted to increase a signal level of the at least one optical signal when the expected noise on the signal path of the at least one optical signal through the network is higher compared to an optical signal having a lower than expected noise on its signal path through the network.
 35. An apparatus for controlling signal launch power of at least one optical signal in an optical communication network, comprising: a) a launcher for launching the at least one optical signal onto the network; and b) means for pre-distorting the launch power of the at least one optical signal in accordance with a known value of a bandwidth of a modulation signal used to modulate the at least one optical signal.
 36. The method as claimed in claim 35, wherein the means for pre-distorting the launch power of the at least one optical signal is also operative for pre-distorting the launch power of the at least one optical signal in accordance with a known value of expected noise on a signal path of the at least one optical signal.
 37. The apparatus as claimed in claim 36, wherein the known values are provided in use by management systems of the optical communication network.
 38. The apparatus as claimed in claim 37, wherein the known values are provided by a network and connectivity information unit.
 39. The apparatus as claimed in claim 37, wherein the known values are supplied by a supervisory channel.
 40. The apparatus as claimed in claim 35, including an optical amplifier through which at least one pre-distorted optical signal is passed in use.
 41. The apparatus as claimed in claim 40, wherein the pre-distorting means includes a comparator for comparing a signal derived from an output of the optical of the optical amplifier with a reference signal dependent on the known value of the bandwidth of the modulation signal used to modulate at least one optical signal.
 42. The apparatus as claimed in claim 36, wherein the expected noise is derived from a number and a type of optical amplifier through which the at least one optical signal will pass in the optical communication network.
 43. The apparatus as claimed in claim 35, wherein the optical communication network is adapted to carry an n channel multiplex, and wherein the launch power of the at least one optical signal is pre-distorted by an optical amplifier in use.
 44. The apparatus as claimed in claim 35, wherein the pre-distorting means is operative for increasing a signal level of the at least one optical signal with an associated modulation signal of a higher bandwidth compared to an optical signal with an associated modulation signal of a lower bandwidth.
 45. The apparatus as claimed in claim 36, wherein the pre-distorting means is operative for increasing a signal level of the at least one optical signal having a higher than expected noise on its signal path through the network compared to an optical signal having a lower than expected noise on its signal path through the network.
 46. The apparatus as claimed in claim 35, wherein the apparatus is an add/drop node. 